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Monday, November 04, 2013

Myths in Biology: Mendel's Pea Flowers

At P.Z. Myers' blog, Pharyngula, he has a post on the subject of "myths" in biology, using the number of cell types in humans as an example. He cites Stephen J. Gould's famous article on fox terriers as an example of such myths and how they get incorporated in the science of biology.

You want myths in biology? Pick up any introductory biology textbook and look up Mendel’s original experiments with garden pea plants. Look at the color illustration of the seven phenotypic characters Mendel supposedly tested. Is purple versus white flower color in the figure? Now, read Mendel’s paper describing the seven different characters in garden peas that he studied (you can find the original paper here, in the original German and in English translation). Is purple versus white flower color in the list of characters tested? Interesting…

Toward the end of the paper, Mendel mentions that in a later set of experiments he tested flower color and found the same ratios that he found with the original seven traits he tested. According to the paper, he tested “violett-rothe und weiss Blüthenfarbe” (i.e. “violet-red and white blossom color”), but this test was NOT in his original set of seven experiments, which are the ones always illustrated in biology textbooks.

So, when did biology textbooks start this particular myth? As far as I can tell it was in the first biology textbook with full color illustrations: William T. Keeton’s Biological Science, 2nd ed. The illustrator thought he could kill two birds with one stone by illustrating the technique used to ensure controlled fertilization (i.e. removing the stamens from the flowers using iris scissors) and purple and white flower color. Except that Mendel didn’t study purple versus white flower color in his original series of seven crosses.

Which pair of traits did Mendel actually study, but were replaced by purple versus white flowers in all introductory biology textbooks? The color of the seed-coat, in which gray-brown is dominant and white is recessive. According to Mendel’s original paper, gray-brown seed coats are associated with (what we would now refer to as linked with) “violet-red blossoms and reddish spots in the leaf axils,” but once again Mendel did NOT explicitly test purple versus white flower color in the experiments for which he is remembered, and for which the science of “Mendelian genetics” is named.

And why is the clearly incorrect list (and colored figure) of the seven traits Mendel supposedly studied included in every introductory biology textbook today? Because Keeton’s textbook was the most widely used textbook in biology for decades, so all of the other publishers simply copied what was in his textbook as a way of gaining market share.

Can this myth be corrected now? How many professors’ sets of lecture notes and PowerPoint slides would have to be changed to correct this mistake, and how many textbooks would need new illustrations that included the correct list of the seven traits, and how many people would complain about these changes, or (even worse) suggest that Mendel really did study purple and white flower color in his original series of seven experiments?